Dalmation Molly swimming issues

What is the water volume of the tank? 10 Gal
How long has the tank been running? ~2 years
Does it have a filter? Yes
Does it have a heater? Yes
What is the water temperature? 80F
What is the entire stocking of this tank? (Please list all fish and inverts.)
2 Female Dalmation Mollies, 2 Female Lyretail Mollies
3 Mystery snails, various pod snails (unintended, brought in from plants)

Maintenance
How often do you change the water? Weekly
How much of the water do you change? 50% or more
What do you use to treat your water? Prime, before it gets put into the tank
Do you vacuum the substrate or just the water? Vacuum substrate too

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? API Liquid testing kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 5-10 ppm
pH: 7.5

Feeding
How often do you feed your fish? Once daily
How much do you feed your fish? pinch, ground up
What brand of food do you feed your fish? Omega One Freshwater flakes
Do you feed frozen or freeze-dried foods? No

Illness & Symptoms
How long have you had this fish? 3 months (March 2020)
How long ago did you first notice these symptoms? 1 day
In a few words, can you explain the symptoms? The Black Dalmation Molly is swimming erratically and staying at the bottom of the tank. Often the fish will just stay at the bottom of the tank and sit there motionless until it is disturbed. There does not look to be bloating and it is swimming upright. Sometimes it ventures up to swim with the other Molly fish. The top and bottom fins look clamped and could explain the erratic swimming (to swim it has to wiggle itself). The other 3 Mollies are fine and show no unhealthy symptoms.

Have you started any treatment for the illness? I've dosed Paraguard for the past two days. No signs of it working.
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? See above.

Explain your emergency situation in detail.
Initially I started with 2 female Dalmation Mollies in early March. One of them showed a bit of aggression towards another (White Dalmation chasing Black Dalmation around the tank, however there was no fin nipping that I saw). One week ago I got two female Lyretail Mollies in order to try to reduce the stress on the bullied Black Molly. The four fish have been acting fine and swimming together until today. The Black Dalmation Molly is hanging around the bottom of the tank and being motionless unless disturbed. She does swim up to eat the food, but then swims back down to the bottom. The other 3 Molly fish are doing well.

There's no exterior abnormalities I saw on the Black Dalmation or other fish, but I did dose paraguard twice (once a day) to be sure. I have a theory that the two Lyretails I recently got are not both female and have impregnated the female Dalmations, so I posted pics of the Lyretails. I read to look for the fan shaped analfin to identify a female, but maybe I messed up and a more experienced hobbyist can correct me. Thanks for any help you all can provide!

White Dalmation Molly (The Bully):




Black Dalmation Molly (The Bullied):
GIF of Black Dalmation Molly swimming erratically

The two female Lyretail Mollies:

 

Mollies are very active and get to about 4". They are probably unhappy in a 10g, they need about 29g.
They don't look pregnant so that's not it.

 

Shimmying is a symptom rather than a single disease, and an indication that a fish no longer has proper control of its nerves and muscles. It occurs when fish are under severe stress, most often because of environmental problems.

When mollies are kept in soft or acidic water conditions, though tolerant fish in many ways, they do not do well, it's very common to see molly's shimmy in such conditions.

 

Shimmying is a symptom rather than a single disease, and an indication that a fish no longer has proper control of its nerves and muscles. It occurs when fish are under severe stress, most often because of environmental problems.

When mollies are kept in soft or acidic water conditions, though tolerant fish in many ways, they do not do well, it's very common to see molly's shimmy in such conditions.

Thanks for the explanation. For further information, I tested my GH and KH levels, they are:
GH: 125.3 ppm
KH: 4 (degrees?) 71.6 ppm

Tested using the API liquid tests. I saw on the test kit it was slightly low for mollies. When I first got the Mollies in March I added a handful of crushed coral to the hang on the back filter, and I went and added more crushed coral after testing tonight. I suppose I’ll keep dosing Paragard and see what the KH and GH are over the next few days?

 

Try adding aquarium salt, 2 tablespoons. Shimmying usually indicates they're struggling to maintain osmotic balance and salt will help.

 

I keep my mollies in my 90g at a ph of 8.0. They didn't do well when I first got them because not knowing, my tap has a ph of 6.8 so I have to adjust my aquarium water.

Like others said above, 10g is much too small for mollies. To put it in perspective, I have a big black molly that's almost 5" . so they're capable of getting rather large for a livebearer. They also enjoy what I found is groups. I have 4 different kinds in my 90g. Theres 5 of each kind. They all seem to enjoy being with all the others. However i try to have more females than males to keep aggressive behavior at a minimum. I'd consider a larger aquarium if you're able to do so.

 

Mollies are very active and get to about 4". They are probably unhappy in a 10g, they need about 29g.
They don't look pregnant so that's not it.

I keep my mollies in my 90g at a ph of 8.0. They didn't do well when I first got them because not knowing, my tap has a ph of 6.8 so I have to adjust my aquarium water.

Like others said above, 10g is much too small for mollies. To put it in perspective, I have a big black molly that's almost 5" . so they're capable of getting rather large for a livebearer. They also enjoy what I found is groups. I have 4 different kinds in my 90g. Theres 5 of each kind. They all seem to enjoy being with all the others. However i try to have more females than males to keep aggressive behavior at a minimum. I'd consider a larger aquarium if you're able to do so.

I was concerned back when my betta passed months ago that the tank would be too small for mollies. I read around and some sites were saying it's too small and others said one or two in 10 gal were just fine. But I agree with you two now, and I'll look to get a larger tank. My only concern is finding a table sturdy enough but not too costly to support +200 pounds in a 20 gal or +300 pounds in a 30 gal.

Try adding aquarium salt, 2 tablespoons. Shimmying usually indicates they're struggling to maintain osmotic balance and salt will help.

Thanks, and I will keep this in my back pocket. I do have mystery snails I'd like to keep alive, and I've heard aquarium salt can hurt snails (something about dissolving it first before adding it to the tank can help?).

Update: The Black Dalmation Molly looks a lot better this morning. No more shimmying, and her dorsal and bottom(?) fins are out. I'll still look to get a larger tank soon. I'll attach a gif of her. Thanks everyone and I'll keep a close eye on her over the next few days!
Hopefully out of the woods!